1. Field of the Invention
The present invention relates in general to a toner supply roll for transferring a toner, and more particularly to a toner supply roll incorporated in an image developing device used in an image forming apparatus such as copying apparatus, image recording apparatus, printer and facsimile, and a method suitable for producing such a toner supply roll. The image developing device is adapted to develop an electrostatic latent image into a visible image consisting of a toner or developer. The visible image is formed on a suitable image bearing medium such as a photoconductive or photosensitive medium used in electrophotography, or a dielectric medium used in electrostatic recording. The toner supply roll functions to transfer the toner to such an image bearing medium for developing the latent image into the visible image.
2. Discussion of the Related Art
In such a known copying, recording, printing, facsimile reception or other image forming apparatus, an electrostatic image formed on a photoconductive or electrostatic dielectric image-bearing medium is developed by an image developing device into a visible image by transfer of a toner to selected local spots on the imagewise exposed image-bearing medium. The image developing device has a hopper accommodating a mass of the toner (developer), and incorporates an image developing roll, and a toner supply roll which is a soft elastic roll adapted to supply the toner to the image developing roll so as to transfer the toner to the image-bearing medium.
For instance, the toner supply roll used in such an image developing device is an elastic roll of a polyurethane foam or sponge structure, as disclosed in JP-A-3-155575. Several methods have been proposed to produce or manufacture such an elastic roll. These methods include: method A including the steps of obtaining a roll by cutting a slab of a foam product generated by foaming a material in a mold, inserting a metal shaft through the roll, and grinding or polishing the circumferential surface of the roll to finish the roll into the desired shape; method B including the steps of foaming a material in a mold so as to form a cylindrical sponge structure on a metal shaft, and grinding the sponge structure to remove an unnecessary portion for thereby obtaining the toner supply roll of the desired shape; and method C wherein a sponge structure is formed on a metal shaft in the same manner as in the method B, but the grinding step is not implemented.
However, the conventional methods A, B and C of producing the elastic roll suffer from various potential problems. For example, the methods A and B include the complicated process steps, and suffer from fluffing of the surface of the roll due to the grinding step, namely, generation of undesirable burrs or fuzz left on the ground or polished surface of the roll; and unsatisfactory dimensional accuracy of the roll. Although the method C is free from such problems, this method does not permit the skin layer of the roll to have a sufficiently large thickness. The insufficient thickness of the skin layer may cause easy breakage of the skin layer due to friction resistance during use of the roll as the toner supply roll, in which the roll is held in rolling contact with an image developing roll. Thus, the method C does not assure sufficient durability of the roll.
The durability of the elastic roll produced according to the method C may be increased by: 1) increasing the density of the sponge structure and increasing the thickness of the skin layer, so as to increase the strength of the skin layer, 2) improving the physical properties (tensile strength, elongation and hardness) of the roll, or 3) employing a so-called "integral skin foam" which facilitates the formation of the skin layer. These measures, however, all result in increasing the hardness of the foam or sponge structure of the roll. Generally, the toner supply roll is required to have a high degree of flexibility as well as a high level of durability. The method C does not permit these two requirements to be satisfied simultaneously.
The elastic toner supply roll of the image developing device is required to have functions of supplying a suitably controlled amount of the toner to the image developing roll and of scratching off an unnecessary amount of the toner from the image developing roll, so that the toner is uniformly distributed on the image developing roll. The surface of the toner supply roll produced according to the known methods A and B tends to be fluffed or given burrs or fuzz, leading to instability of the amount of the toner to be transferred to the image developing roll, and resulting in deteriorated quality of an image reproduced by the toner. Further, the burrs removed from the toner supply roll may act as foreign matters which may be unfavorably left in the other portions of the image forming apparatus, resulting in the deteriorated quality of the reproduced image and malfunction of the apparatus.
The elastic toner supply roll produced according to the known method C suffers from the problem of foreign matters as indicated above with respect to the methods A and B, namely, removal of fragments of the material of the sponge structure due to breakage of the skin layer of the sponge structure as described above. Further, the toner is likely to enter the interior of the sponge structure through the broken portions of the skin layer, resulting in hardening of the broken portions, that is, local hardening of the sponge structure of the roll, which may cause instability of the amount of the toner to be transferred from the roll.
The conventionally used toner which is transferred by the toner supply roll tends to be required to have a relatively small particle size and a relatively low melting point, so as to meet demands for an improved image quality reproduced by the toner and an increased speed of printing. Such a toner is likely to be aggregated due to electrostatic charging and long-term storage thereof. Accordingly, the aggregated masses of the toner powder are likely to remain as films of the toner ("toner filming" defect) on the outer circumferential surface of the image developing roll, so that the remaining toner films cannot be sufficiently scratched off by the toner supply roll from the surface of the image developing roll, leading to occurrence of an unfavorable variation of the toner concentration or density of the reproduced image, which may cause a ghost image ("ghosting" defect). The toner supply roll is held in rolling contact with the image developing roll, and is rotated with the image developing device in the same direction. At the nip between two rolls, the toner supply roll removes the residual toner stuck to the surface of the image developing roll, while evenly transferring a new layer of the toner to the surface of the image developing roll. However, the conventional elastic toner supply roll is not sufficiently highly capable of scratching off the residual toner which is aggregated or stuck on the outer circumferential surface of the image developing roll, resulting in partial remaining of the toner on the outer circumferential surface of the image developing roll. This may cause uneven distribution of the toner on the image developing roll, resulting in a variation of the toner concentration of the reproduced image and reproduction of a ghost image.
For improving the function or capability of the toner supply roll to scratch off the residual toner stuck on the image developing roll, it may be considered to increase the hardness of the toner supply roll or to increase the contact pressure at the nip between the toner supply roll and the image developing device. Both of the proposed measures are effective to improve the scratching function of the toner supply roll, but unfavorably increase the contact pressure between the toner supply roll and the image developing roll, leading to tearing or wearing of the image developing roll, deterioration of the particles of the toner (grinding of the toner particles), and excessive electrostatic charging of the toner. Therefore, these measures may cause deterioration of the quality of the reproduced image, during long-term use of the roll, such as reduction of the toner concentration and undesirable transfer of the toner to local portions of the recording medium at which no image should be printed. It may also be considered to increase the size of cells of a sponge structure of the toner supply roll. In this case, the toner is likely to enter the inside of the sponge structure thorough the cells, resulting in hardening of the sponge structure at the local portions where the toner is entered, whereby the quality of the reproduced image may be deteriorated.